DOC: Add CONTRIBUTING and PSC docs

Migrate contributors guide and PSC status to two new files at the root of the main repository. README.md is also updated to reflect this.

After merge of this MR I propose to delete these wiki pages:

• https://wiki.orfeo-toolbox.org/index.php/Project_Steering_Committee
• https://wiki.orfeo-toolbox.org/index.php/Decision_making
• https://wiki.orfeo-toolbox.org/index.php/Contributors_guidelines
• https://wiki.orfeo-toolbox.org/index.php/Commit_message_flags

I have also added this new wiki page which is linked from CONTRIBUTING.md:

• https://wiki.orfeo-toolbox.org/index.php/Remote_Modules

Also, I think this page should be renamed to "Release policy" and well updated (or just deleted), but I have not done anything about it now:

• https://wiki.orfeo-toolbox.org/index.php/Git

work done together with @ytanguy